// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/base/moving_average.h"

#include <algorithm>

namespace media {

MovingAverage::MovingAverage(size_t depth)
    : depth_(depth)
    , count_(0)
    , samples_(depth_)
    , square_sum_us_(0)
{
}

MovingAverage::~MovingAverage()
{
}

void MovingAverage::AddSample(base::TimeDelta sample)
{
    // |samples_| is zero-initialized, so |oldest| is also zero before |count_|
    // exceeds |depth_|.
    base::TimeDelta& oldest = samples_[count_++ % depth_];
    total_ += sample - oldest;
    square_sum_us_ += sample.InMicroseconds() * sample.InMicroseconds() - oldest.InMicroseconds() * oldest.InMicroseconds();
    oldest = sample;
}

base::TimeDelta MovingAverage::Average() const
{
    DCHECK_GT(count_, 0u);

    // TODO(dalecurtis): Consider limiting |depth| to powers of two so that we can
    // replace the integer divide with a bit shift operation.

    return total_ / std::min(static_cast<uint64_t>(depth_), count_);
}

base::TimeDelta MovingAverage::Deviation() const
{
    DCHECK_GT(count_, 0u);

    const double size = std::min(static_cast<uint64_t>(depth_), count_);
    const double average_us = total_.InMicroseconds() / size;
    double sqr_deviation_us = square_sum_us_ / size - average_us * average_us;
    if (sqr_deviation_us < 0)
        sqr_deviation_us = 0;

    return base::TimeDelta::FromMicroseconds(sqrt(sqr_deviation_us));
}

void MovingAverage::Reset()
{
    count_ = 0;
    total_ = base::TimeDelta();
    square_sum_us_ = 0;
    std::fill(samples_.begin(), samples_.end(), base::TimeDelta());
}

} // namespace media
